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-   -   Average static pressure BCs and aircore in cyclone (http://www.cfd-online.com/Forums/cfx/24932-average-static-pressure-bcs-aircore-cyclone.html)

KM December 17, 2007 02:31

Average static pressure BCs and aircore in cyclone
 
Hi,

I am simulating flow in hydrocyclone. I have following two difficulties and would be very much thankful if some one can throw some light on it:

1) I set the relative pressure inside my domain equal to 1 atm. As both the outlets (overflow and underflow) are open to atmosphere, the gause pressure at these outlets should be set as Zero Pa. I guess I should prefer the 'Average static pressure' boundary condition for these outlets rather than 'static pressure' as the flow is swirl.

Moreover, the 'Average Static Pressure' condition implements the zero gradient at the boundary and hence experts suggest to put the outlet boundaries far away from the region of interest. In other words, I should extend the length of overflow and underflow outlet pipes. So my question is about the appropriateness of applying zero pressure at these extended outlets as I will be modeling extra length of these pipes. Is it still fine to apply zero pressure at the outlets?

2) An air core exists around the central axis of cyclone because of its outlets open to atmosphere. Now in my case, air core will not exist but I need to validate some existing results for my studies. All the data which I can find, seems to have air core existence. I got an idea to approximate the air core diameter and cut an equivalent cylinder from my geometry and give FREE SLIP water velocity at its surface which would represent the air-water interface. How good this approximation should be? Any comments?

Thanks a lot for helping me out... I have gained a lot from the contributers to this forum,

Regards, KM.

Glenn Horrocks December 17, 2007 16:34

Re: Average static pressure BCs and aircore in cyc
 
Hi,

Point 1) Do you mean relative pressure or absolute pressure? You probably want to set the reference pressure to be one atmosphere and the outlet to zero relative pressure. Also it is a good idea to extend the domain away from the chamber a little as you say so the details of the boundary conditions do not affect the main chamber.

Point 2) A multiphase model means you don't have to do this. Alternatively you can define the shape of the free surface with a multiphase model and use that as the fixed shape for future single phase models, providing the free surface does not move much.

Glenn Horrocks

KM December 17, 2007 20:50

Re: Average static pressure BCs and aircore in cyc
 
Dear Glenn,

Thank you very much for your reply. :)

Regarding Point 1):

Say my outlet was actually located at X=a, Y=b, Z=c and I want to specify my boundary condition as 'Average Static Pressure'=0. I extend my outlet pipe and now let us assume that my new outlet is located at X=a+L, Y=b, Z=c. Hence the relative pressure = 0 will be applied at X=a+L rather than X=a. Hence the pressure at X=a will be different than zero.

My case is incompressible and I understand that the pressure drop will matter, not the absolute value. But what if I specify my inlet also as a pressure boundary conditions.

Am I able to put my doubt well?

Thank you very much ,

KM


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